Lighting of a high-intensity discharge lamp (HID bulb) used for a vehicle headlamp requires a high-voltage generating apparatus called “an igniter (IGN)” in general, and the igniter generating a high voltage uses a high-voltage generating transformer. For conventional high-voltage generating transformers for a discharge lamp lighting apparatus, the following conventional examples are listed up.
As a conventional example 1, the example is a high-voltage generating transformer fabricated by winding a primary winding part on a secondary winding part formed of rectangular electric wires edgewise wound around a bar core, and a feature of the example is that the secondary winding part formed of edgewise wound rectangular electric wire is assembled directly on a core of high-resistance Ni—Zn system ferrite, thus enabling the external shape of the winding part to be small and the axial length thereof to be shortened.
Further, the embodiment thereof shows that a variety of methods of winding a primary winding part can be employed, and also shows an example employing traverse-winding of rectangular electric wire by using insulated round copper wire and a primary winding bobbin, or an example using thin conducting foil insulated with film; however, in any of those ideas, the primary winding part is disposed biased toward the low voltage side of the secondary winding part (see Patent Document 1, e.g.).
In a conventional example 2, there is shown a high-voltage generating transformer having a structure similar to that of the above-described conventional example 1 (Patent Document 1), and the conventional example 2 treats the core thereof as a conductor in contrast to the conventional example 1 treating the core as an insulator. For this reason, the conventional example 2 requires having insulation between the core and the secondary winding part, and in order to cause the secondary winding bobbin to have an insulating property, the bobbin on the high voltage generating side of the secondary winding part is thickened. However, those conventional example 1 and conventional example 2 have the same arrangement in the fundamental portion of the primary winding part and the secondary winding part (see Patent Document 2, e.g.).
A conventional example 3 is a high-voltage generating transformer where a secondary winding part formed of round wire is wound on a bobbin divided into some sections and a primary winding part is wound around the outer periphery of the secondary winding part, and a feature of the conventional example 3 is that a structure is employed where a core having a hole at the center is used and a high-voltage output terminal penetrates through the core.
The conventional example 3 shows in its embodiment as with the conventional example 1 that various methods of winding the primary winding part can be used, and also shows an example where traverse-winding of rectangular electric wire is adopted by using insulated round wire and a primary winding bobbin; however, the example says that optimum disposition of the primary winding part is concentrating the winding part in the central section and even roughly wound winding part using a bobbin can give an excellent characteristic as long as the winding part is axially evenly disposed (see Patent Document 3, e.g.).
As a conventional example 4, the example is a high-voltage generating transformer intended for a high-intensity discharge lamp not using mercury as described later, and the transformer is fabricated by winding a primary winding part over a secondary winding part formed of rectangular electric wire wound on a bar core as with the conventional example 1. Lighting a high-intensity discharge lamp not using mercury requires a high current, and a feature of the conventional example 4 is that in order to reduce the secondary winding part in its axial length, having its thickness increased because of its cross-sectional area enlarged for flowing the high current, the secondary winding part is divided, the secondary winding part of the low voltage side is wound on the outside of the secondary winding part of the high voltage side, and further, the primary winding part is provided on the outside of the part of the low voltage side.
Moreover, the embodiment thereof also shows an idea where the wall of the bobbin functioning as a partition wall for obtaining a withstand voltage is formed in a stepwise shape or wedge shape in cross section according to the applied voltage (see Patent Document 4, e.g.).
Patent Document 1: JP-A-2002-093635
Patent Document 2: JP-A-2005-322515
Patent Document 3: JP-A-2001-257087
Patent Document 4: JP-A-2004-111451
Conventional high-voltage generating transformers used for discharge lamp lighting apparatuses are arranged as mentioned above, and are actualized and commercialized by using some methods.
Among those transformers, both high-voltage generating ones shown in the conventional example 1 (Patent Document 1) and the conventional example 3 (Patent Document 3) meet high-intensity discharge lamps (HID bulbs) using mercury (referred to as “conventional bulbs” hereinafter), and those technologies concerning the transformer are sufficiently completed for high-voltage generating transformers used for igniters.
However, thereafter, high-intensity discharge lamps not using mercury (referred to as “Hg-free bulbs” hereinlater) have begun to be put to practical use in consideration of mercury that is environmental material. Since an Hg-free bulb is energized with bulb current approximately two times greater than a conventional bulb is, the technology meeting conventional bulbs cannot sufficiently function by itself in designing a new igniter for an Hg-free bulb as follows. Therefore, the igniter therefor should be further enhanced in performance.
For example, although the rated powers of a conventional bulb and a Hg-free bulb for an on-vehicle headlamp are both 35 W; the rated current of an Hg-free bulb is 0.8 A; the rated current of a conventional bulb is 0.4 A; the rated voltage of an Hg-free bulb is 42 V; and the rated voltage of a conventional bulb is 85 V.
Therefore, in the winding part of a high-voltage generating transformer used for an igniter for an Hg-free bulb, it is necessary to reduce the electric resistance of the winding part by a factor of four in order to cope with the approximately doubled current and make the heat generation caused by the loss equivalent to the heat generation in the conventional bulb. For this reason, supposing that the diameter of the electric wire used for the winding part is simply doubled (four times in cross-sectional area), the winding part increases in volume, the portion housing the winding part provided on the bobbin also thereby increases, and the igniter increases in size and cannot be accommodated in the housing space of the headlamp, which can cause a size problem.
In order to cope with the size problem, even if the shapes of parts surrounding the igniter can be changed and the igniter increased in size can be thereby attached to the headlamp, all vehicles do not always have space for the expansion, and there is a possibility that the igniter cannot be used in another vehicle. This causes the igniter to lose its compatibility among varied vehicles, and results in causing the igniter to deteriorate its marketability as a component for vehicles.
Further, a winding part using thick electric wires puts a large distance between neighboring winding parts, and causes its magnetic flux making an interlinkage to leak, thus weakening its magnetic coupling. This deteriorates its electric performance, and also causes a characteristic problem.
As described above, a high-voltage generating transformer intended for an Hg-free bulb requires a heavy-current specification as compared with that of a transformer intended for a conventional bulb. Cases of trying to cause each of the above-stated conventional examples to accommodate the high-current specifications will be explained hereinafter.
In the conventional example 1 (Patent Document 1), in order for the example to meet the above-described high current, the rectangular electric wire constituting the secondary winding part needs to be enlarged in width or thickness; however, in the structure using the secondary winding part where rectangular electric wire is edgewise wound as described above, there is a limitation in the ratio of the width to the thickness of rectangular electric wire capable of being edgewise wound. It is impossible to enlarge the wire only in width while maintaining the wire in the thickness required for a conventional bulb and at the same time keeping the transformer in the total length required therefor. It is required to enlarge the wire in width and simultaneously increase the wire also in thickness. Accordingly, if a high-voltage generating transformer is built depending on the type of the conventional example 1 by using rectangular electric wire having an increased cross-sectional area because of being increased in thickness and width for energizing the transformer with a high current, the secondary winding part increased in thickness and width increases the high-voltage generating transformer in the diametrical direction and the axial direction (in the longitudinal direction of the core), and the transformer increases the igniter in size. There is a problem that an igniter having the same size as that of an igniter intended for a conventional bulb cannot be fabricated.
In the case of the conventional example 2 (Patent Document 2), in order to pass a high current the rectangular electric wire constituting the secondary winding part is increased in thickness, which increases the distance from the primary winding part to the secondary winding part (especially the secondary winding part located on the side opposite from the primary winding part) (the core is also elongated). For this reason, the magnetic flux generated by the primary winding part becomes easy to leak from halfway the elongated secondary winding part. If the magnetic flux generated by the primary winding part leaks from partway the secondary winding part, the magnetic flux generated by the primary winding part does not reach the portion of the secondary winding part located remotely from the primary winding part. The portion of the secondary winding part that the magnetic flux does not reach does not function as the part of a transformer. Therefore, there is a problem that the igniter pulse voltage generated on the side of the secondary winding part becomes low, and thereby, the high-voltage generating transformer cannot deliver sufficient performance.
In the conventional example 3 (Patent Document 3), also in the structure such as that of this conventional example, where the primary winding part is wound around the outer periphery of the secondary winding part formed of round wire, divided into some sections, the wire shape of the secondary winding part is compelled to be enlarged in order to meet a large current applied to an Hg-free bulb.
Accordingly, also in the high-voltage generating transformer based on the conventional example 3, when electric wire having a heavy shape in cross section for passing a large current is used, there occurs a problem that the igniter increases in size as with the case of the conventional example 1, and moreover, there occurs a problem of performance described later.
To say more exactly, the secondary winding part constituting the high-voltage generating transformer expands in the direction of coil diameter (in the diametrical direction), and is axially elongated, which increases the distance from the primary winding part to the secondary winding part in the diametrical direction (to the layer of the secondary winding part located in the central portion) and in the axial distance. The increase of the distance between the primary winding part and the secondary winding part causes the magnetic flux generated by the primary winding part to easily leak from halfway the radially expanded winding part or from halfway the axially elongated winding part. When the magnetic flux generated by the primary winding part leaks from halfway, there occurs a problem that sufficient output cannot be obtained as with the case of the conventional example 2.
In the conventional example 4 (Patent Document 4), the example is intended for meeting an Hg-free bulb as previously stated, and is disclosed with the intention of further enhancing the performance of an igniter by solving the above-mentioned problem occurring in supporting an Hg-free bulb, to be more precise, the problem of size of the igniter accommodated in a limited space for vehicle use, or the problem of electric performance (characteristic) deterioration of a high-voltage generating transformer.
However, when the secondary winding part is divided between the low voltage side and the high voltage side and the divided portions of the winding part are placed in superposed relation, a bobbin for isolation for obtaining the withstand voltage between the high voltage section and the low voltage section is used, and further, the primary winding part is disposed biased toward one side of the secondary winding part. For this reason, it is inevitable that the distance from the primary winding part to the secondary winding part will be increased, the structural problem that the magnetic flux generated by the primary winding part easily leaks from halfway the secondary winding part is not so improved, and there is a possibility (problem) that the obtained transformer cannot deliver sufficient performance as a high-voltage generating transformer.
The above explanations are given to the cases of trying to cause the conventional examples 1-4 to meet high-current requirements.
About the leak of the generated magnetic flux described in the above-mentioned problem occurring in meeting an Hg-free bulb, in order to prevent the magnetic flux from leaking through the clearances between lines of the primary winding part, and also prevent the magnetic flux from leaking through the clearance between the secondary winding part and the primary winding part by keeping both the winding parts in close contact with one another, it can be said that the structure of a high-voltage generating transformer having a three-layer structure where the secondary winding part is divided between two bobbins in the direction of coil diameter (in the diametrical direction) and the primary winding part is inserted between the secondary winding part divided into two portions is most suitable. Further, the three-layer structure reduces the axial length thereof.
However, in the high-voltage generating transformer having the above-mentioned three-layer structure where the axial length can be reduced, there occur problems that the radius of the convolution of the secondary winding part increases because the diameter of the winding part expands and the length of the secondary winding part required for the convolution increases to thereby increase the resistance of the secondary winding part, thus resulting in no enhancement of characteristic, and further, increasing the igniter in size.
As mentioned hereinabove, if the structures based on the conventional examples are applied to fabricating the high-voltage generating transformer used for the Hg-free bulb newly commercialized, the problems of increase in the size of an igniter and deterioration in the characteristics of the igniter cannot be solved.
The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a high-voltage generating transformer for a discharge lamp lighting apparatus, which does not excessively increase an igniter in size, reduces the leak of the magnetic flux generated from a primary winding part to cause the magnetic flux to make an interlinkage with a secondary winding part, and has characteristics meeting the lighting of an Hg-free bulb.